1. Field of the Invention
The present invention relates to a system which prevents an aircraft, such as a helicopter, from traveling at a speed exceeding a limit (and in particular, a lateral speed in the rightward or leftward directions, or a rearward speed), as well as assisting in flying the helicopter so as to avoid areas of strong wind turbulence, by simultaneously measuring the three-axis components of the airspeed of the aircraft, and the three-axis components of the wind speed ahead of the aircraft, and displaying or presenting this information to the pilot or the automatic control system controlling the aircraft.
2. Description of the Related Art
In many cases, when a helicopter is stationary in the air (hovering), then although it has a capability for traveling sideways or rearwards, restrictions are placed on the speed of this travel (lateral speed in left or rightward direction, rearward speed) due to the flight characteristics. However, a conventional type of airspeed sensor, based on a Pitôt tube system (pressure measurement sensor), is not capable of measuring lateral speed or rearward speed, and therefore, although the pilot can judge the speed of the aircraft on the basis of the relative movement of the ground terrain, and the handling characteristics of the aircraft, it is difficult to judge the speed in this way in bad weather conditions, or at night time, when the terrain is difficult to see, or when the aircraft is over the sea, where there are few objects that serve as indicators. Hence, there is a danger of going beyond the speed restrictions. Moreover, a helicopter may encounter air disturbances relating to the local topography, for instance, when used to transport goods or perform rescue tasks in mountainous areas, or when landing at or taking off from heliports on high-rise buildings, and in such cases, the capability to present information on the wind speed in front of the helicopter, to the pilot, in advance, would contribute towards increasing safety and improving the probability of completing tasks successfully. In view of these points, there is a need for a system which is capable of measuring the three-axis airspeed including the lateral speed and the rearward speed components, in a helicopter, and measuring the wind speed information in front of the helicopter. Furthermore, it is also necessary to display information relating to the measured airspeed and wind speed, to the pilot, in a readily comprehensible fashion.
One known method for measuring the speed of movement of the atmosphere (wind speed) at a remote point, is a Doppler anemometer. A Doppler anemometer is a device which irradiates a beam of radio waves, light waves (laser) or acoustic waves, into the atmosphere, and determines the remote wind speed from the Doppler shift in the resulting scattered wave. However, the wind speed which can be measured by this device is limited to a one-axis component in the direction of irradiation of the beam. Depending on the type of beam irradiated, Doppler anemometers are classified as radar anemometers (which irradiate radio waves), lidar anemometers (which irradiate laser light), and sodar anemometers (which irradiate sound waves).
In recent years, systems have been manufactured for obtaining wind speed information in front of the aircraft, or the airspeed of the aircraft, by mounting a lidar anemometer in an aircraft. The present applicants have also discovered a system of this kind, and have acquired patent rights for same in Japan and the U.S.A. The invention relating to a “Wind Turbulence Prediction System” in the U.S. Pat. No. 6,751,532 has the object of providing a wind turbulence prediction system which is capable of measuring the three-dimensional wind turbulence and confirming in advance whether or not an issued warning is reliable, and in order to achieve this system, a lidar anemometer is mounted in the aircraft, a laser beam is irradiated while being scanned in a conical shape, and the scattered light scattered by the regions of wind turbulence ahead of the aircraft in flight is received, thereby achieving a method which measures the speed of the remote three-dimensional air flow. Furthermore, in order to account for the effects of vertical winds and fore and aft winds on the aircraft, the measured three-dimensional air flow information is converted into vertical wind only, and displayed in a simplified, two-dimensional fashion, in such a manner that the wind turbulence is represented separately in terms of the strength of the turbulence and the average wind speed. However, the system for measuring the three-axis components of the wind speed ahead of the aircraft by means of a lidar anemometer of this kind does not consider the use of information on the airspeed of the aircraft. Moreover, there is no compensation for the movement of the aircraft during laser beam scanning, and hence there is a problem in that the error becomes large when the aircraft is moving in an unsteady fashion, or moving at high-speed. In addition, the display of the obtained wind speed information is restricted to a display of the wind conditions ahead of the aircraft, with respect to a passenger aircraft which does not vary flight path largely. However, in the case of a helicopter, there may be large vertical or lateral changes in the flight path when traveling at low speed, and therefore, the wind speed information in the vertical and lateral directions is also required, in addition to information on the wind speed in the forward direction of the aircraft.
Furthermore, at the “30th European Rotorcraft Forum” held in Marseilles, France on September 2004, the presentation given by “Thales Avionics” and “ONERA” on “A1.5 μm LIDAR demonstrator of low airspeed measurement for civil helicopter” related to a system for measuring the three-axis components of the airspeed of the helicopter by means of a lidar anemometer, whereby the three-axis components of the airspeed is acquired by irradiating and scanning a laser beam in a conical fashion. However, it does not provide compensation for the movement of the aircraft during scanning, and therefore has large error when the aircraft is performing unsteady movements or traveling at high speed. Moreover, the laser beam is irradiated in an upward lateral direction from the aircraft, and there is a problem in that the wind speed ahead of the aircraft cannot be measured.
These systems are restricted to measurement of the wind speed ahead of the aircraft, and measurement of the airspeed of the aircraft, respectively, and no mention is made of acquiring and displaying these two information elements simultaneously. Furthermore, no compensation is provided in respect of the movement of the aircraft during the travel of the beam of the lidar anemometer, and the error becomes large when the aircraft performs unsteady movement or travels at high speed.